In most color video input and output devices, each primary color that is being scanned or displayed is processed separately. For example, in a color television camera, special optics that include appropriate color filters separate incoming light into its red, green, and blue components, which are then separately imaged onto three monochrome camera sensors. In a projection television display, three Cathode Ray Tubes (CRTs) or Liquid Crystal Display (LCD) light valves are used to separately generate monochrome red, green, and blue images, which are then combined together using special optics. In a computer display or television receiver with a color CRT, three separate electron beams are used to excite red, green, and blue phosphors on the screen.
In order to properly reproduce a complete video image, each of the red, green and blue component color images must be properly aligned with respect to the other such images, so that all these component images will, to a viewer, fuse together into a single displayed color image. This alignment is called color registration or color convergence. In general, color registration is difficult to achieve because of the effects of optical system geometry, optical errors, alignment errors, and nonlinearities in the camera or display devices themselves. For example, in magnetically deflected camera sensors and CRTs, difficulties inherent in precisely and accurately generating proper magnetic fields result in different geometric errors for each primary color. The geometric errors alter a positional correspondence among the primary color images, and therefore adversely affect the color registration. For example, in a 13 inch (approximately 33 cm) (diagonal) television or computer display, a one percent geometric distortion error in only a red channel image will cause portions of the red image to be shifted approximately 0.08" (2 mm) with respect to green and blue channel images. Such a displacement is readily apparent to a viewer. In general, color convergence is never perfect everywhere in the image, and the degree of alignment will generally vary significantly throughout the image. A common practice is to adjust for optimum convergence at the center of the image, i.e., as close to perfect as can be achieved there. Elsewhere the degree of alignment will generally degrade towards the sides and corners of the image, often reaching a maximum at some point along the periphery. This lack of color registration and convergence is called color misregistration or misconvergence.
The effects of color misregistration adversely affect an image in various ways, for example, elements drawn in secondary colors (combinations of the primary colors) will have color variations that are called color fringes. For example, referring to FIGS. 1B and 1C, yellow line 14 drawn on a CRT display screen with misregistered red and green components 15 will have red fringe border 17 on one side, central yellow portion 16, and green fringe border 18 on the other side. As such, the image containing such a line will appear to the viewer to be blurred or out of focus. Furthermore, there will also be a loss of detail in the displayed image because the misregistered components are positionally shifted into adjoining areas, which, in turn, broadens local image detail and causes it to overlap with other image detail. When the misregistration is greater than the line width, separate offset images will appear. These problems are especially serious for High Definition Television (HDTV) and computer displays, where fine image detail, graphics and text characters are often viewed closely by a user.
Most prior art methods for detecting color misregistration use a crosshatch and dot pattern that are drawn, e.g. on a CRT display screen, in a single secondary color. Such a pattern is shown in FIG. IA. As shown, the pattern contains vertically ruled lines, collectively identified as lines 11, which are useful for detecting color fringing in the horizontal direction, horizontally ruled lines, collectively identified as lines 12, which are useful for detecting color fringing in the vertical direction, and dots, of which dot 13 is illustrative, which are useful for detecting color fringing in either direction. In use, a trained operator examines an image containing a displayed crosshatch and dot pattern for color fringing and makes appropriate adjustments to minimize the misalignment. The procedure is generally performed for each of the secondary colors: yellow, cyan, and magenta in order to test for red-green, blue-green, and red-blue misalignment. With such a crosshatch and dot pattern, the color fringes tend to be difficult to detect because they are often just slivers of light that are not as bright as a centrally aligned secondary color. The primary colors also have different visibilities, for example, blue is difficult to see next to green or red. Also, because of a smooth Gaussian profile of the electron beams used in the CRT display, a misregistration produces a gradual smooth transition to and from the color fringes, making these fringes more difficult to spot than an abrupt and sharp transition. As such, since the operator must scan the image for rather minute color fringes, this process for detecting misregistration using a crosshatch and dot pattern tends to be quite slow and exhibits a low sensitivity to misconvergence and misregistration.
Many automated methods for measuring misregistration separately and successively determine centroids for key areas of a monochromatic test pattern in each of the red, green, and blue primary colors. See, for example, U.S. Pat. Nos. 4,635,095 and 4,988,857. These methods are generally slow and require expensive test equipment. Moreover, a time lag between a sequential measurement of the centroids of each separate color image introduces inaccuracies because of instabilities and drift in the scanning circuits of the camera or display. Other methods actively shift or offset a scanning pattern in order to measure misconvergence. See, for example, U.S. Pat. Nos. 4,642,529 and 4,686,429. These latter methods require that: (a) expensive test equipment be used, and (b) various modifications be made to the scanning circuits in a display device in order for these methods to be used. Since the scanning pattern will necessarily be altered, misregistration tests based upon these latter methods cannot be performed on a device that has not been so modified, i.e. a device that is in normal working order.